Field of the Invention
The present invention relates to a scanning optical device of an image forming apparatus that forms an image by scanning an image bearing member with a laser light beam and, more particularly to a scanning optical device and an image forming apparatus including a photodetector that detects a laser light beam on an upstream side of an image bearing member and generates a synchronization signal.
Description of the Related Art
As the scanning optical device used in the image forming apparatus of this type, there has been known, for example, a scanning optical device described in Japanese Patent Application Laid-Open No. H11-187219.
The scanning optical device includes a rotary polygon mirror that repeatedly deflects a laser light beam from a semiconductor laser at a fixed angle and scans and exposes a photosensitive member surface, which is an image bearing member, in a direction parallel to the rotation axis of a photosensitive member and a scanner motor that drives to rotate the rotary polygon mirror.
A photodetector that detects the laser light beam is provided in a region further on an upstream side than an image writing start position on the photosensitive member. A synchronization signal for synchronizing image writing start timing is generated from an output waveform of the photodetector.
In the generation of the synchronization signal, the output waveform of the photodetector is changed to a square wave at a predetermined threshold, a center position of a signal width is detected from timings of both of rising and falling of the square wave, and the synchronization signal is output at timing of a half position from the position of the falling. By setting the center of the signal width as a reference in this way, even if the signal width is different depending on a light amount of the laser light beam, it is possible to output the synchronization signal at the same timing.